In the manufacture of electric cables, particularly those having a conductor insulated with a multi-layer covering, for use at elevated temperatures under extreme pressure and exposure to oil and water environments, it has been found useful to employ an insulation comprising a thermosetting rubber containing ethylene and propylene usually referred to as an "ethylene-propylene rubber" or "EP rubbers". The ethylene-propylene rubber may be a copolymer, though more commonly it is an ethylene-propylene-diene-monomer terpolymer which, in turn, is usually referred to as an "EPDM". Such cables are disclosed, for example, in U.S. Pat. No. 4,088,830, and typical insulation formulations compounded from such EPDM polymers together with additional constituents, such as suitable processing aids, curing agents and the like, have been shown, for example, in U.S. Pat. No. 3,926,900. In particular, it is accepted practice to add so-called processing oils to retard swelling from contact with hot fluids and well gases.
Suitable EP rubbers and EPDM polymers are available from a variety of sources included UNIROYAL under the trademark ROYALENE and E. I. Du Pont under the trademark NORDEL. The final formulation or "compound", however, typically will also include curing agents, processing aids, fillers, pigments and/or cross-linking agents. In general, the selection of a particular curing system, or other additive, is usually a matter of choice for the individual formulator subject to certain well-known characteristics attributable to specific additives.
In formulating compounds from such ethylene-propylene rubber compositions, it has been necessary to strike a balance between the desire for compounds having better physical characteristics, i.e. modulus, hardness etc., and the counterbalancing highly undesirable changes in other physical properties. For example, it is generally accepted in EP rubber compounding that higher modulus compounds can be obtained by using carbon black or precipitated silica fillers. However, these fillers are unsuitable for cable applications because they seriously degrade the electrical properties. In other words, even though it is known that compounds having the desired higher modulus could be obtained using carbon black or precipitated silica fillers, in cable applications, it has been traditional to use clay as the filler because the electrical properties are the principal properties which must be maintained. Similarly, while it has long been desired to further retard swelling from well fluids, the addition of more processing oil to achieve such an end will also result in a lowering of modulus, hardness, and other physical properties.
Nevertheless, it has long been desired to formulate an EP rubber based compound which would have better resistance to swelling due to contact with well gases and hot fluids, without the heretofore commensurate reduction in modulus and hardness, so that the resulting insulation would still be resistant to mechanical deformation at room temperature. Such insulation deformation frequently occurs during the cable armoring process, as well as when the cable is handled during service.
It has also long been desired to produce a cable with a multi-layer covering over the conductor wherein the thermosetting rubber insulation has not only excellent electrical insulation properties and physical properties, such as modulus and hardness; but, also improved resistance to swelling after exposure to well gases and hot fluids.
Accordingly, it is a principal object of the present invention to provide an electrical cable of the type used in oil and gas wells which has not only excellent physical and chemical properties, but also improved resistance to exposure to well gases and hot fluids.
In addition, other objects of the invention will become apparent to those skilled in the art from a reading of the following specification and claims.